1. Field of the Invention
The present invention relates to a thin film device including a thin film laminate structure such as a thin film transistor (hereinafter referred to as a TFT) and a method for making the same, and in particular relates to a thin film device capable of low cost production due to a decreased initial investment and a method for making the same. Also, the present invention relates to a liquid crystal panel and an electronic device using the thin film device.
2. Background Technology
In recent years, liquid crystal display devices using such types of thin film devices have been used in notebook-type personal computers, car navigation systems, video cameras and various portable information devices, and their range of applications and production is drastically increasing. Such phenomena are due to improved performance including reduced price of the liquid crystal display devices, enlarged screen size, improved image resolution and low electrical power consumption. Further cost reduction is, however, required for further expansion of the market and range of applications.
The mainstream of the liquid crystal devices is active matrix liquid crystal devices using TFTs as switching elements for pixels. Each liquid crystal device includes TFTs, a TFT substrate on which a matrix of pixel electrodes connected to the TFTs are formed, a counter substrate provided with a common electrode, and a liquid crystal encapsulated between these two substrates. FIG. 17 shows the main section of a TFT substrate 60. In FIG. 17, TFTs 61 are formed at pixel positions near the intersections of a plurality of source or data signal lines S1, S2, . . . Sn arranged in columns with a plurality of gate or scanning signal lines G1, G2, . . . Gm arranged in rows. Source electrodes of the TFTs 61 are connected to their respective data lines, and drain electrodes are connected to their respective pixel electrodes 62. The data signal supplied from a data line is applied to a pixel electrode 62 through its corresponding TFT 61 based on the scanning timing signals supplied through the corresponding gate line. The state of the liquid crystal is changed and driven for displaying by an electric field between the pixel electrode 62 and the common electrode, not shown in the drawing.
The liquid crystal display device is fabricated by panel assembling including encapsulation of the liquid crystal between the TFT substrate 60 and the counter electrode, and packaging of driving circuits for driving the data lines and the gate lines. The cost of the liquid crystal display device greatly depends on the cost of the TFT substrate 60. The cost of the TFT substrate 60 depends on the manufacturing method of the TFTs. A part of driving circuits may be formed on the TFT substrate 60 by forming the active elements with the TFTs, and in this case, the cost of the TFT substrate represents a high proportion of the cost of the liquid crystal display device.
A TFT has a thin film monolithic structure including a plurality of thin films which include at least a silicon semiconductor layer having an insulating layer, a conductive layer, a source, a drain and a channel region. The cost of the TFT greatly depends on the production cost of the thin film monolithic structure.
The insulation layer in the thin film monolithic structure is formed by a low pressure chemical vapor deposition (LPCVD) process or a plasma enhanced CVD (PECVD) process, because a normal pressure CVD (NPCVD) process results in low uniformity of the film thickness. The conductive layer, or typically the metal layer, is formed by a sputtering process. The silicon film for forming the silicon semiconductor layer is also formed by the PECVD or LPCVD process. Further, a method for implanting an impurity into the silicon film by an ion implanting process or an ion doping process is used. Alternatively, the high concentration impurity region which functions as a source-drain region is formed of an impurity-doped silicon film in a CVD system.
The CVD systems and the sputtering system used in the above-mentioned film deposition processes belong to vacuum units for processing materials under vacuum pressures, and require large vacuum systems, resulting in an increase in initial investment. In the vacuum system, a substrate is transferred to a vacuum evacuation chamber, a substrate heating chamber, a film deposition chamber and a vent chamber, in that order, to form a film. The substrate atmosphere therefore must be changed from open air to vacuum, and this limits the throughput. Because the ion implanter and the ion-doping system are also vacuum systems, the same problems as above occur. Further, the ion implanter and the ion-doping system require complex mechanisms for generating plasma, extracting ions, mass-separating the ions (for the ion implanter), accelerating ions, collimating ions, scanning ions and so on, resulting in a remarkably high initial investment cost.
As described above, the thin film deposition technology and the processing technology for producing a thin film monolithic structure are basically similar to the manufacturing technology for LSI circuits. The main means for cost reduction of the TFT substrate include scaling-up of the substrate size for forming TFTs, improvement in efficiency of the thin film deposition and its processing step, and improvement in yield.
Scaling-up of the substrate size for producing large liquid crystal display devices with reduced costs is an obstacle to high speed transfer of the substrates in the vacuum system, and causes breakage of the substrate due to thermal stress during the deposition steps, hence it is significantly difficult to improve the throughput of the film deposition system. Also, the scaling-up of the substrate size inevitably requires scaling-up of the film deposition system. An increased cost accompanied by the increased volume in the vacuum system further increases the initial investment, and as a result, it is difficult to achieve drastic cost reduction.
Although an increased yield is a valuable means for cost reduction, a yield near the limit has been achieved, and thus drastic cost reduction is difficult in view of the yield.
Patterning of each layer is performed by a photolithographic process. The photolithographic process essentially includes a coating step, an exposure step and a developing step of a resist film. After these steps, an etching step and a resist-removing step are required, hence the steps for patterning is a factor in increasing the number of steps for thin film deposition. This is a factor in the increased cost of thin film device production.
Regarding the resist-coating step in the photolithographic process, only less than 1% of the resist solution dropped onto the substrate remains on the substrate as the resist film after spin coating, reducing the efficiency of the use of the resist solution.
Although a printing process has been proposed as a low cost process instead of a large scale exposure system used in the exposure step, it has not yet reached practical use due to problems such as processing accuracy.
As described above, it is not possible to drastically reduce the cost of the TFT substrate, although the market requires drastic price reduction of the liquid crystal display devices.
It is an object of the present invention to provide a thin film device and a method for making the same, in which a part, or all of, the films in a thin film monolithic structure used for a liquid crystal display device are deposited without a vacuum system in order to decrease initial investment and operation costs, increase the throughput and significantly decrease the production costs.
It is another object of the present invention to provide a thin film device and a method for making the same, in which a thin film having characteristics similar to those of a CVD or sputtered film is formed of a coating film while achieving cost reduction.
It is a further object of the present invention to provide a thin film device and a method for making the same, in which the consumption of a coating solution is decreased in the formation of the thin coating film for achieving cost reduction.
It is still another object of the present invention to provide a thin film device and a method for making the same, which is capable of patterning the formed film without a photolithographic process and, thus, reducing the cost.
It is a still further object of the present invention to provide a thin film device, a liquid crystal panel and an electronic device using the same, in which a plane in contact with the liquid crystal can be planarized by forming a pixel electrode with a coating film.
It is another object of the present invention to provide a thin film device, a liquid crystal panel, and an electronic device using the same, in which a wiring layer can be used as a light-shielding layer for a black matrix and the thin film device has a high aperture ratio.
It is still another object of the present invention to provide a liquid crystal panel and an electronic device which enable cost reduction due to use of an inexpensive thin film device.
According to an embodiment of the present invention, a thin film device has a thin film monolithic structure comprising a plurality of thin films including at least one insulating layer and at least one conductive layer, wherein
at least one thin film in the thin film monolithic structure is formed of a coating film (excluding a spin-on-glass film having a basic structure comprising siloxane bonds), which is obtained by applying a solution containing a constituent of the thin film followed by annealing.
A method for making the thin film device comprises the following steps of:
applying a coating solution containing a constituent of the thin film onto a substrate; and
forming a coating film (excluding a spin-on-glass film having a basic structure comprising siloxane bonds) by annealing the coated surface of the substrate.
In the present invention, at least one layer in the thin film monolithic structure is formed as a coating film without a vacuum system. As such a coating film, a spin-on-glass (SOG) film having a basic structure comprising siloxane bonds, which has been used as a planarization layer, has been known. The organic SOG film is, however, readily etched during an oxygen plasma process, whereas the inorganic SOG film readily cracks even if the film has a thickness of several thousand angstroms, hence it is rarely used solely as an interlevel insulating film, and is used as only a planarization layer above a CVD insulating film.
In the present invention, an insulating layer and a conductive layer composing a thin film monolithic structure are formed of a coating film other than the SOG film, and the thin film can be planarized at the same time. Because the coating film can be formed without a vacuum system such as a CVD system or a sputtering system, a mass-production line can be constructed with a significantly smaller investment compared to conventional systems, the throughput of the system can be increased, and the cost of the thin film device can be drastically reduced.
The thin film monolithic structures include various structures, for example, those including semiconductor layers, those including thin film transistors, and those including an underlying insulating layer and an upper protective insulating layer.
In these cases, it is preferable that all of the insulating layers contained in the thin film monolithic structure be formed of a coating film. A gate insulating layer requiring a critical film quality for ensuring desired thin film transistor characteristics, however, may be formed by a method other than a coating process.
It is preferable that at least two thin films in the thin film monolithic structure be formed by a coating process in order to reduce the device cost which is a purpose of the present invention.
The insulating layer can be formed of a SiO2 coating film, which is obtained by applying a solution containing a polymer having Sixe2x80x94N bonds (polysilazane), followed by a first annealing process in an oxygen atmosphere. Because the polysilazane having the above structure exhibits high cracking resistance and oxygen plasma resistance, a single layer can be used as an insulating layer having a given thickness.
It is preferable that the insulating layer be subjected to a second annealing process at a temperature higher than that in the first annealing process to further clean its surface. The second annealing process may be performed at a high temperature for a short period using a laser or a lamp.
The semiconductor layer is formed by implanting an impurity into a silicon coating film, which is formed by applying a solution containing silicon particles, followed by a first annealing process.
It is preferable that the semiconductor layer be subjected to a second annealing process at a temperature higher than that in the first annealing process to improve the crystallinity in the layer. The second annealing process may also be performed at a high temperature for a short period using a laser or a lamp.
Preferably, a method for diffusing an impurity into the silicon coating film comprises the following steps of:
forming by coating an impurity-containing layer onto the silicon coating film; and
diffusing the impurity into the silicon coating film by heating the impurity-containing layer.
Conventionally, the high concentration impurity region which functions as a source-drain region has been formed of an impurity-doped silicon film by a CVD system, or formed by introduction of an impurity by an ion implanting process or an ion doping process. In the present invention, a source-drain region is formed by a step of applying and baking a solution to form a thin film containing an impurity, and by a step of annealing the thin film at a high temperature for a short period using a lamp or a laser to form a high concentration impurity region. The ion implanting system and the ion doping system basically belong to vacuum systems, and require extremely complicated mechanisms for generating plasma, extracting ions, mass-separating the ions (for the ion implanter), accelerating ions, collimating ions, scanning ions and so on. Hence these two systems have evidential high prices compared to the system for coating and annealing the thin film containing the impurity.
There are two methods for forming the conductive layer. In one method a thin metal film is formed and in the other method a thin transparent conductive film is formed.
The formation of the thin metal film as a conductive layer includes coating of a solution containing conductive particles and then evaporating the solvent by a first annealing process. A conductive coating film can be thereby formed.
It is preferable that the conductive layer also be subjected to a second annealing process at a temperature higher than that in the first annealing process to reduce the resistance of the layer. The second annealing process may be performed at a high temperature for a short period using a laser or a lamp.
Preferably, a method for forming a transparent conductive film as a conductive layer comprises:
a first annealing step annealing the coated surface in an oxygen or nonreductive atmosphere; and
a second annealing step annealing the coated surface in a hydrogen or reductive atmosphere.
When forming the transparent electrode as the conductive layer, for example, an organic acid containing indium and tin is used as a coating solution. Preferably in this case, a solvent used for adjusting the viscosity is evaporated (at, for example, a temperature of approximately 100xc2x0 C.) after coating, and then the above-mentioned first and second annealing processes are performed. Indium oxide and tin oxide are formed during the first annealing process, and the film is reduced during the second annealing process in a hydrogen or reductive atmosphere.
It is preferable that the temperature in the second annealing process be lower than that in the first annealing process.
The transparent conductive coating film after the first annealing process can be prevented from thermal deterioration in the second annealing process.
Preferably, the substrate is maintained in the nonoxidizing atmosphere after the second annealing process until the substrate temperature is decreased to 200xc2x0 C. or less. The reoxidation of the transparent conductive coating film reduced during the second annealing process can be thereby suppressed, and thus the sheet resistance of the transparent conductive coating film does not increase. It is preferable that the substrate be introduced into open air at a temperature of 100xc2x0 C. or less in order to ensure prevention of the reoxidation. Because the resistivity of the coated ITO film decreases in proportion to the oxygen defects in the film, the reoxidation of the transparent conductive coating film due to oxygen in the open air results in an increase in the specific resistivity.
In the formation of the transparent conductive coating film, a coating solution containing indium (In) and tin (Sn) is applied onto the substrate. The coating film is oxidized in the first annealing process to form an ITO film. Using the coated ITO film, the conductive layer is also usable for the transparent electrode.
When the surface of the ITO film is plated with a metal, the film can be used as a conductive layer other than the transparent electrode, and the metal plating can decrease the contact resistance.
It is preferable that a conductive sputtering film be formed on the contact face of the coated ITO film by a sputtering process.
An example of the thin film monolithic structure is an active matrix substrate including pixel switching elements provided on their respective pixels, which are formed near intersections of a plurality of data lines with a plurality of scanning lines, and pixel electrodes connected thereto.
A typical pixel switching element used in the active matrix substrate is a thin film transistor. The thin film transistor as the pixel switching element includes a gate electrode electrically connected to one of the scanning lines and a drain electrode electrically connected to one of the pixel electrodes.
It is preferable that the pixel electrodes be formed of a conductive coating film in such a thin film monolithic structure. The surface in which the pixel electrodes are formed generally has steps, while the surface of the conductive coating film is substantially planarized when the pixel electrode is formed of the conductive coating film. As a result, rubbing can be satisfactorily performed and occurrence of reverse-tilt domains can be prevented.
It is preferable that the conductive coating film used for the pixel electrodes be a coated ITO film. The coated ITO film functions as a transparent electrode and is suitable for producing an active matrix substrate in a transmission liquid crystal display device.
The thin film transistor as the pixel switching element includes an interlevel insulating film formed on the front surface of the gate electrode, and the data line and pixel electrode are electrically connected to the source region and the drain region, respectively, through contact holes formed in the interlevel insulating film.
The interlevel insulating film may be composed of a lower interlevel insulating film which lies at the lower side, and an upper interlevel insulating film which is formed on the surface of the lower interlevel insulating film. In this case, the data line is electrically connected to the source region through a first contact hole formed in the lower interlevel insulating film. On the other hand, the pixel electrode is electrically connected to the drain region through a second contact hole formed in the lower interlevel insulating film and the upper interlevel insulating film.
In such a configuration, the data line and the pixel electrode are formed on different layers from each other, hence these do not short-circuit each other even if they are formed at a position in which they overlap with each other. The periphery of the pixel electrode can therefore be arranged above the data line and the scanning line.
As a result, no planar gap is present between the data line or scanning line and the pixel electrode. The data line and the scanning line can therefore function as a black matrix having a light-shielding function. Accordingly, it is not required to form a light shielding layer as the black matrix by an additional process.
Because the range capable of forming the pixel electrode is expanded, the aperture ratio of the pixel region is increased, resulting in a bright display.
It is preferable that the pixel electrode formed of a conductive coating film be electrically connected to the drain electrode through a conductive sputtering film.
Because the sputtering film has a lower contact resistance than that of the conductive coating film, the contact resistance can be reduced by positioning the conductive sputtering film between the conductive coating film and the source region.
It is preferable the conductive sputtering film be a sputtering ITO film so as not to decrease the aperture ratio.
When the conductive coating film and the conductive sputtering film have the same pattern, the accuracy in the patterning of the pixel electrode can be improved, because a resist film can be formed on only the conductive coating film having high adhesiveness to the resist mask, and the conductive coating film and the conductive sputtering film can be simultaneously patterned. Resist mask formation on the conductive sputtering film having low adhesiveness to the resist mask is not required, and a decrease in accuracy in the patterning can be avoided.
When the conductive coating film and the conductive sputtering film do not have the same pattern, it is preferable that the periphery of the conductive coating film lies outside of the periphery of the conductive sputtering film.
Resist masks are separately formed on the conductive coating film and the conductive sputtering film and are separately subjected to sputtering by different steps. The accuracy of the patterning for the periphery of the pixel electrode depends on the accuracy of the patterning for the conductive coating film having a larger patterning dimension than that of the conductive sputtering film. The low accuracy of the patterning for the conductive sputtering film having low adhesiveness to the resist mask does not affect the accuracy of the patterning for the pixel electrode.
When the conductive sputtering film and the data line are present in the same layer, these can be simultaneously formed of the same metal material.
Alternatively, the conductive sputtering film may lie above the data line. In this case, as these layers are formed by different steps, these layers may be formed of the same material or different materials.
The interlevel insulating film may include a lower interlevel insulating film at the lower side and an upper interlevel insulating film deposited on the surface of the lower interlevel insulating film, and the data line and the conductive sputtering film may be formed on the surface of the upper interlevel insulating film. The data line is electrically connected to the source region through a first contact hole formed in the lower interlevel insulating film. On the other hand, the conductive sputtering film is electrically connected to the drain region through a second contact hole formed in the upper interlevel insulating film and the lower interlevel insulating film. The conductive coating film is deposited on the surface of the conductive sputtering film.
Alternatively, the data line and the conductive sputtering film may be formed in the same layer on the surface of the lower interlevel insulating film. In this case, the data line is electrically connected to the source region through a first contact hole formed in the lower interlevel insulating film. The conductive sputtering film is electrically connected to the drain region through a second contact hole formed in the lower interlevel insulating film. Further, the conductive coating film is deposited on the surface of the upper interlevel insulating film, and electrically connected to the conductive sputtering film through a third contact hole formed in the upper interlevel insulating film.
In accordance with another embodiment, a liquid crystal panel comprises:
an active matrix substrate provided with the above-mentioned thin film device,
a counter substrate facing the active matrix substrate, and
a liquid crystal layer encapsulated between the active matrix substrate and the counter substrate.
In accordance with a further embodiment, an electronic device comprises the liquid crystal panel.
In these cases, the cost reduction in the thin film device enables drastic cost reduction of the liquid crystal panel and the electronic device using the liquid crystal panel.
In the above-mentioned solution coating step, it is preferable that the solution be applied to only the coating region on the substrate to form a patterned coating film on the substrate, because a photolithographic process including many steps is not required. According to this process, consumption of the coating solution decreases and thus the operation cost can be reduced.
In accordance with still another embodiment of the present invention, a method for making a thin film device is characterized in that a patterned coating film is formed on the substrate by:
preparing a coating solution dispenser head provided with a plurality of liquid discharging nozzles, and
discharging the coating solution onto only the coating region on the substrate while relatively changing the positions of the substrate and the liquid discharging nozzles.
This method can be achieved by, for example, an ink jet process. Because the coating solution is not wasted and no photolithographic process is required, this method greatly contributes to the investment cost reduction and improved throughput. For example, in conventional coating techniques only approximately 1% of a dropped resist has been used as a coating film, whereas in the present invention 10% or more of a dropped resist can be used as a coating film. Of course, such a high coating efficiency holds for the other coating films in the present invention, and thus the reduced use of the coating materials and the reduced time in the coating processes enable the cost reduction of liquid crystal display devices.
It is preferable that these nozzles be independently controlled to discharge or not to discharge the coating solution, and positions of the substrate and the discharge nozzles be relatively changed while controlling the coating timing on the nozzle. More precise pattern coating can thereby be achieved.
Such a coating process is applicable to coating of various coating solutions for forming coating films by other than coating of the resist for forming a resist pattern. For example, if an insulating coating film is pattern-coated, a contact hole can be formed simultaneously with the coating.
As described above, in accordance with the present invention, a part or all of the thin films can be formed by applying and annealing a solution, hence a thin film device can be produced with an inexpensive production unit having a high throughput.